1. Field of the Invention
Generally, the present invention is directed to novel, linear polyester resins. The present invention also is directed to energy curable lithographic inks and coatings including the novel, linear polyester resins. The present invention is further directed to articles of manufacture printed with lithographic inks or coatings.
2. Description of the Related Art
Energy curable lithographic inks have been used for many years in printing and graphic art technologies. The commercial appeal of using energy curable lithographic inks stems from its improved physical and chemical resistance properties post-cure. For example, in commercial application when lithographic images are printed and cured on substrates such as cartons or cardboard problems associated with ink scratching and flaking can be reduced and/or eliminated during downstream substrate folding processes.
Considerable resources have been spent by ink manufacturers to improve the overall characteristics of energy curable lithographic inks. Properties including but not limited to dot gain, water window and misting are of particular importance to lithographic inks. Dot gain commonly is defined as the mechanical increase in half tone dot size which occurs as the image is transferred from the plate to the blanket of a printing device, and ultimately transferred onto paper. Larger dot gain values, e.g., unintended spreading, may result in poor image quality. Water window commonly is defined as an ink's tolerance of water or fountain solution. Wide water window values are indicative of an ink's ability to sustain high print density. Alternatively, narrow water window values are indicative of low print density. Low print density inks cause toning, piling, scumming, and excessive dot gain.
Misting is another problem associated with lithographic inks. Misting is defined as the expulsion of small ink droplets from printing nips due to high printing speeds and low ink structure. Misting poses health and clean-up problems contrary to good manufacturing practices (GMPs). Namely, ink droplets form aerosol which ultimately migrates onto printing equipment or other sensitive areas in a print room. In addition, ink droplets in the form of aerosol may unintentionally be inhaled by employees causing unforeseen health concerns.
Regarding energy curable lithographic ink formulations, monomers or oligomers such as epoxy acrylates and polyester acrylates have been employed for improving certain print characteristics. For example, Bisphenol-A epoxy acrylates and their analogues, have been used in low performance inks to improve tack and misting. Tack is defined as the measured separation force in an ink film required for proper transfer and printing onto a substrate. For high performance inks, polyester acrylates and their analogues are most often employed becomes of their lower viscosities and higher functionalities. While acrylate monomers or oligomers are useful in reducing misting, they have proven ineffective in obtaining wide water windows, e.g., low water or fountain solution tolerance. This is attributed to their reactivity which poses stability concerns for ink formulations based on their polarity and solubility profiles.
To control misting and dot gain, gelled or structured varnishes (resins) have been employed in energy curable lithographic inks. Gelled varnishes are produced from resins reacted with chelating agents such as aluminum alkoxides and include coordinated covalent and hydrogen bonds. However, gelled resins are often incompatible with reactive acrylate-based monomers and oligomers. Namely, the polar properties of acrylates cause a breakdown of hydrogen bonding structures in gelled varnishes. To compensate, the gelled varnishes typically are enhanced with very high levels of gellant. However, poor flow and transfer qualities arise in energy curable lithographic inks resulting in handling difficulties. The gellants may also cause poor ink printability attributed to low water emulsification properties.
According to the inventors, there is still a need in the art for a novel polyester resin which improves printing properties of energy curable lithographic ink formulations. There is also a need in the art for energy curable lithographic ink formulations exhibiting wide water window values, improved print contrast, improved print density, and improved rheology (body) characteristics.